1) Start installing Moodle on the system without curl PHP extension.
2) After accepting license agreement
Confirm you can see "Server checks" page (/admin/index.php?cache=0&lang=en&agreelicense=1). Page must not be blank or contain messages about PHP errors.
Confirm you see the table row with the following cells: "php_extension", "curl" "must be installed and enabled".
Confirm you see "cURL PHP extension is not installed" message in "Other checks" table.

Description

Steps to reproduce:
1) Start installing Moodle on the system without curl PHP extension (in my case it was a Debian Linux box without php5-curl package installed).
2) After accepting license agreement you get redirected to "Server checks" page (/admin/index.php?cache=0&lang=en&agreelicense=1).

Expected behavior:
The page should load properly and display the information about missing curl extension.

Actual behavior:
In production environment this page is blank (because of disabled error reporting in PHP). In development environment with E_ALL error reporting level this page displays the following error:

PHP Fatal error: Call to undefined function curl_version() in /path/to/moodle/lib/upgradelib.php on line 2334